When Big Blind Mistakenly Fold His Cards...

Hi... i am trying to find an answer about a strange situation. Maybe someone can help me.
In a NLHO tournament, final table, three players playing... SB posts 500 and BB 1000. BB sees his cards and folds imediately (by mistake) , but the hand is declared dead. Third player folds too. SB in his turn has nothing else to do and fold his cards taking BB's 1000. TD decides that SB should take just 500, and give back 500 to BB, saying that SB called just the 500. Is this a correct decision????